Orbiter will deploy its radar to hunt for underground water.
The radar stowed on board the European Space Agency's Mars Express orbiter will finally be unfolded in early May.
The Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) will look for traces of water ice beneath the martian surface, and could potentially detect reservoirs up to five kilometres underground.
Mars Express arrived in orbit around the red planet on 25 December 2003, and MARSIS was supposed to have been deployed in April 2004.
But computer simulations by the instrument's manufacturers, Astro Aerospace of Carpinteria, California, showed that when the radar's long booms swing into position they might hit other instruments or fatally destabilize the craft.
MARSIS consists of a pair of 20-metre hollow fibreglass cylinders, each 2.5 centimetres in diameter, and a 7-metre boom. Wires inside the tubes will generate radio waves that penetrate planet's crust before reflecting back to a receiver onboard Mars Express. The tubes are folded into a concertina that will automatically stretch out when its container is opened.
The boom system was created by NASA's Jet Propulsion Laboratory in Pasadena, California, which has been running a full assessment of the risks of unfolding MARSIS. An independent engineering review board of the European Space Agency and industry experts finally decided on 25 January that the tubes posed little threat to Mars Express, and the instrument should be unfurled in the first week of May.
"We're all slightly jittery about this because of the predictions," says John Murray from the Open University in Milton Keynes, UK, who works on Mars Express's High Resolution Stereo Camera. "Nobody really wants to take the risk, except the MARSIS team," he says.
If all goes to plan, MARSIS will probe the planet until at least 30 November, the proposed end date for the Mars Express mission.
"It's a tremendously important experiment," says Murray. "There's so much evidence now of water flowing on the surface in the past, you wonder how much might still be there."
The MARSIS results could also direct future robotic and manned missions to the surface, adds Murray: "If there are vast reservoirs of ice, that's the sort of place we have to look for life."
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Peplow, M. Mars Express goes for boom or bust. Nature (2005). https://doi.org/10.1038/news050207-5